Flotation apparatus, often called flotation machines or flotation cells, are commonly employed to separate solid material (e.g. ore) from slurry, which is typically composed of liquids and solids in varying proportions. A flotation machine usually has a tank with a rotor therein and some form of gas delivery system. In use, the rotor rotates and agitates slurry in the tank dispersing gas, from the gas delivery system, thereby causing the formation of gas bubbles.
Typically the slurry comprises at least a hydrophobic material which is separated from the slurry by adhering to the gas bubbles, floating to the surface, and forming a froth at the surface that has a higher concentration of the adhered material than the slurry. The froth, being a combination of liquid, solid particles, and gas, is then removed for further processing.
The gas bubble to particle interaction is important to the process as without it, there can be no separation using the described flotation method. The rotor is considered to be one of the most important aspects of the flotation machine, and in achieving the gas bubble to particle interaction, as the other components merely react to the movement of the rotor.
There are three functions in particular which the flotation machine should achieve. The first is solid suspension. Virtually all flotation machine installations are utilized for the separation of slurry and, according, it is vital that the solids are kept suspended within the liquid because otherwise the gas bubbles cannot collide with the particles to carry them upward. Furthermore, if the solids build up to any degree, the volume of the cell is reduced and retention times and short circuiting can occur. A build up of solids can also eventually overwhelm the rotor and stop the cell from working altogether.
The second function is air dispersion. The amount of energy required to suspend solids is considerably less than it is to disperse air. For example, a typical flotation machine would use 300 kW to process a 300 m3 tank, with the suspension of a typical slurry estimated to require only a 30 kW portion of that power.
Finally, the third function is circulation. The contents of the flotation cell have to be well circulated to ensure that solid particles come into contact with the dispersed gas as often as possible. This ensures the solid particles have ample opportunity to adhere to the gas bubbles, and consequently assists in getting optimal recovery of the solid material.
Known flotation cells have a rotor with impeller blades located inside a tank within which the slurry is received and processed. The rotor typically has a hollow shaft which transports a gas to an outlet located on or near the rotor. A horizontal baffle plate is typically located at or near the top of the impeller blades to disperse the gas across the width of the rotor. The impeller blades are typically curved in profile, following an arc tending towards the axis of rotation of the rotor such that the rotor has a smaller diameter at the bottom than at the top.
There are many disadvantages associated with these rotor arrangements, resulting in reduced performance and efficiency. For example, the gas leaves the shaft, and enters the slurry under the rotor baffle plate where it travels horizontally underneath the baffle plate to its perimeter. At the periphery of the baffle plate the air mixes with the slurry in a high shear contact region. As this region is only at the top of the rotor blades, after which the gas typically travels upwards away from the rotor, it is relatively small. This inefficiently disperses the gas in the slurry, also often resulting in irregular bubble sizes as large amounts of the gas can escape the high shear zone and form bubbles that are too large to adhere to solid particles.
Furthermore, the baffle plate on top of the impeller blades prevents vertical movement of flows into the rotor and, therefore, circulation in the tank is limited, particularly above the baffle plate. In order to try to overcome this problem, some attempts have been made to introduce further impeller blades half way up the shaft of the rotor.
A common problem in flotation cells of the above design is ‘sanding’. Sanding occurs when the solids collect and build up at the bottom of the tank in a stagnant, or at least very slow moving, layer. Some attempts have been made to reduce sanding problems by increasing agitation above and below the rotor, such as using a guiding element half way down the impeller blades to simultaneously suck the slurry up (from the bottom) and down (from above the rotor): This improves some of the mentioned issues, such as improving circulation above the rotor. However, sanding can still occur as the suction from below has regions of low or no activity that is bypassed by the slurry flow.
These problems, among others, significantly reduce the efficiency and effectiveness of a flotation cell. This increases costs in operating the cell, and reduces the recovery rate of the desirable solids from the slurry.